1. Field of the Invention
The present invention relates to a solid state imaging device, in particular to a CCD (charge coupled device) solid state imaging device.
2. Description of Related Art
Demand for solid state imaging devices as imaging devices of digital still cameras and digital video cameras has been increasing in recent years. Further, in the field of mobile devices such as cellular phones, camera-equipped ones are highly demanded. From this aspect, the demand for the solid state imaging devices has been expanding more and more. The growing demand for the solid state imaging devices involves demand for enhanced image quality. In order to achieve high image quality by the solid state imaging devices, the number of pixels and the S/N ratio have to be increased.
What is required for the increase of the pixel count of a solid state imaging device is to enhance the operation speed of the solid state imaging device. In order to enhance the operation speed of a CCD solid state imaging device, it is necessary to transfer signal charges from an imaging section to a charge accumulating section at high speed.
Aiming at enhanced charge transfer speed, for example, Japanese Unexamined Patent Publication No. H8-236743 proposes a technique of forming shunt interconnections for providing drive pulses to transfer electrodes such that the electric resistance of the transfer electrodes is reduced and a delay in signal charge transfer is reduced.
FIG. 7 is an enlarged plan view of an imaging section of a conventional CCD solid state imaging device having shunt interconnections. As shown in FIG. 7, the imaging section of the conventional CCD solid state imaging device includes a plurality of photoelectric conversion portions 101 formed in a semiconductor substrate in a matrix arrangement and a plurality of vertical transfer channels 124 extending in the column direction between the photoelectric conversion portions 101 adjacent to each other in the line direction. On the vertical transfer channels 124, a plurality of vertical transfer electrodes 111 are formed to extend in the line direction. Each of the vertical transfer electrodes 111 is formed not to overlap the photoelectric conversion portions 101. On the vertical transfer electrodes 111, a plurality of shunt interconnections 114 are formed to extend along the vertical transfer channels 124. The vertical transfer electrodes 111 and the shunt interconnections 114 are insulated from each other by an interlayer insulating film. The shunt interconnections 114 are provided with protrusions 114a protruding in the line direction, respectively. Each of the protrusions 114a has a via plug 129 for electrically connecting the shunt interconnection 114 and the vertical transfer electrode 111. Parts of the photoelectric conversion portions 101 not covered with the shunt interconnections 114 and a light shield film (not shown) serve as light receiving portions.
As the vertical transfer electrodes 111 are electrically connected to the shunt interconnections 114, the electric resistance of the vertical transfer electrodes 111 is reduced and a delay in charge signal transfer is reduced. Thus, the obtained solid state imaging device is operated at high speed.
Although the conventional solid state imaging device achieves high speed operation, there are still problems such as reduction of the ratio of signal to noise (S/N ratio) and variations in sensitivity.
If the via plug for electrically connecting the shunt interconnection and the vertical transfer electrode is formed above the vertical transfer channel, work function of the vertical transfer electrode varies at a portion thereof contacting the via plug. This affects the signal charge transfer badly, e.g., some charges are not transferred properly during the transfer. For this reason, the protrusions of the shunt interconnections are indispensable.
However, when the protrusions are formed, light is reflected on the shunt interconnections irregularly and the reflected light is likely to enter the light receiving portions as stray light. Further, the reflected light is likely to enter other regions than the light receiving portions through gaps between the shunt interconnections and openings in the light shield film. If light entering from invalid regions such as gaps between on-chip lenses formed above the light receiving portions is reflected on the shunt interconnections to become stray light, the presence of the protrusions makes the occurrence of the stray light irregular. As a result, noise increases and the S/N ratio decreases, thereby increasing minimum subject illumination and decreasing effective sensitivity. Further, the light enters pixels different from the target pixel, thereby causing color mixing.
The solid state imaging device is used in combination with lenses. Therefore, light incident on the imaging section is not parallel light but the incident light is more slanted as the light approaches the periphery of the imaging section. Therefore, the sensitivity and smear gradually vary as the light approaches the periphery of the imaging section. If the shunt interconnections are shaped asymmetrically, the variations in sensitivity and smear are affected also by the angle of incident light, thereby making the variations irregular. As a result, obtained images are significantly degraded and correction to them is extremely difficult.